Studies have been carried out on the mechanisms for the formation of lithogenic or supersaturated bile. Multiple types of studies were carried out-in patients on factors regulating the secretion of cholesterol, bile acids and phospholipids in bile as well as bile acid pool sizes. Results indicated that biliary secretion of cholesterol is regulated largely by rates of cholesterol synthesis. Increased secretion rates of cholesterol were found in obese subjects, and these increases were related to enhanced cholesterol synthesis in obesity. The increased cholesterol synthesis and hepatic secretion of obesity is probably responsible for the greater incidence of gallstones in obese subjects. Also, our studies indicate that synthesis rates of bile acids are the major factor regulating the pool size and secretion rates of bile acids. Although the rate of recycling of bile acids may regulate bile acid synthesis to some extent, there was no clear cut relationship between recycling rates and pool sizes of bile acids. Our results also confirm that chenodeozycholic acid is more potent than cholic acid in reducing hepatic cholesterol secretion and lithogenicity of bile. BIBLIOGRAPHIC REFERENCES: Bennion, L.J. and Grundy, S.M.: Effects of obesity and caloric intake on biliary lipid metabolism in man. J. Clin. Invest., 56:996-1011, 1975. Bilheimer, D.W., Goldstein, J.L., Grundy, S.M., and Brown, M.S.: Reduction of cholesterol and low density lipoprotein synthesis after portacaval shunt surgery in a patient with homozygous familial hypercholesterolemia. J. Clin. Invest. 56:1420-1430, 1975.